Method of treating surface of aluminum blank

ABSTRACT

With the purpose of forming an anodic oxide coating that is given conductivity or other new functions on the surface of aluminum-based material with high productivity, anodizing of aluminum-based material ( 2 ) is performed in an anodizing bath containing sulfuric acid together with nitrate ion to form a porous anodic oxide coating on the surface of the aluminum-based material ( 2 ). In another processing step, if electroplating is performed after anodizing, silver or a silver compound or other metal ( 7 ) can be electroplated from an electroplating bath without dissolving and removing the barrier layer from the bottom ( 6 ) of the pores ( 3 ) of the porous anodic oxide coating ( 1 ).

TECHNICAL FIELD

This invention relates to a surface treatment technique whereby metal iselectroplated onto an anodic oxide coating formed on the surface ofaluminum-based materials to give conductivity to said anodic oxidecoating.

BACKGROUND ART

When aluminum-based materials consisting of aluminum or aluminum alloyare subjected to anodizing in a sulfuric acid bath or oxalic acid bath,as shown in FIG. 3(A), a porous anodic oxide coating 1 can be formed onits surface. Such an anodic oxide coating 1 has the function ofincreasing the weather resistance of aluminum-based materials 2, so thisis used widely in a wide range of fields such as building materials anddecorative products and the like.

In addition, as shown in FIG. 3(C), when metal 7 is electroplated ontothe interior of the pore 3 of each cell 4, it is given conductivity soit can have new applications such as crack-resistant anti-staticmaterials. However, a thick barrier layer 5 is formed in the bottom ofthe pores 3 in a porous anodic oxide coating 1 formed by conventionalmethods, so in order to electroplate metal 7 onto the interior of thepores 3 to give it conductivity, as shown in FIG. 3(b), it is necessaryto remove the barrier layer 5 formed in the bottom of the pores 3 andthen perform the electroplating process. In a conventional method usedto remove this barrier layer 5, after anodizing is performed in asulfuric acid bath or oxalic acid bath, the anodizing voltage in thesame electrolyte bath or a different electrolyte bath is graduallylowered over a period of 15 to 20 minutes, thereby electrochemicallydissolving the barrier layer 5 at the bottom of the pores 3 in theanodic oxide coating 1. In another method in use, after anodizing isperformed in a sulfuric acid bath or oxalic acid bath, the power isturned off and the workpiece is left in the same electrolyte bath or adifferent electrolyte bath for a period of 15 to 30 minutes therebychemically dissolving the barrier layer 5 at the bottom of the pores 3in the anodic oxide coating 1. Moreover, both the latter method ofdissolving the layer chemically and the former method of dissolving thelayer electrochemically may also be used together to dissolve thebarrier layer 5.

However, processing takes a long time in any of these conventionalmethods, so they have a problem in that their productivity is low. Inaddition, all of these processes require complicated and specialtechniques, so they have another problem in that their quality isunstable.

To this end, the object of this invention is to provide a surfacetreatment method whereby, without taking a long time in processing, ananodic oxide coating with no barrier layer or with a barrier layer sothin that it exhibits tunneling on the bottom of the pores is formedstably on the surface of aluminum-based materials and then metal iselectroplated onto the interior of the pores in said anodic oxidecoating to give said anodic oxide coating conductivity.

DISCLOSURE OF THE INVENTION

In order to achieve the above object, in the surface treatment methodfor aluminum-based materials according to the present invention,anodizing of an aluminum-based material consisting of aluminum oraluminum alloy is performed in an anodizing bath comprising nitrate iontogether with at least one ion selected from among an organic acid ionor an inorganic acid ion able to form a porous anodic oxide coating,thereby forming a porous anodic oxide coating on the surface of saidaluminum-based material, and then, electroplating of said aluminum-basedmaterial in performed in an electroplating bath comprising metal ion sothat metal is electroplated from said electroplating bath into the poresin said porous anodic oxide coating, thereby giving said anodic oxidecoating conductivity.

In the surface treatment method according to the present invention, theanodizing bath contains nitrate ion together with an organic acid ion oran inorganic acid ion able to form a porous anodic oxide coating, so byusing this anodizing bath to perform the anodizing of the surface ofaluminum-based materials, at the same time that the porous anodic oxidecoating is being grown, the barrier layer is being dissolved at thebottom of the pores. For this reason, by the time that the anodizingprocess is complete, the barrier layer in the bottom of the pores in theporous anodic oxide coating is thin enough to exhibit tunneling or thereis no barrier layer in the bottom of the pores. For this reason, evenwithout performing current restoration or galvanic dissolution or othercomplicated barrier layer removal process in order to remove the barrierlayer from the bottom of the pores in the porous anodic oxide coating,by simply performing the electroplating immediately it is possible toelectroplate metal onto the interior of the pores in the anodic oxidecoating to give the anodic oxide coating conductivity or other newfunctions.

In the present invention, as the aforementioned anodizing bath, it ispossible to use a bath containing, for example, 100 g/l-300 g/l ofsulfuric acid and 7 g/l-140 g/l of nitric acid or a nitrate. Inaddition, as the aforementioned anodizing bath, it is also possible touse a bath containing, for example, 100 g/l-300 g/l of sulfuric acid, 7g/l-140 g/l of nitric acid and 10 g/l-100 g/l of a nitrate.

In the present invention, the anodizing is performed with the anodizingbath at a temperature of 0° C.-30° C. and at a current density of 0.5A/dm²-5.0 A/dm². Here, if the temperature of the anodizing bath isroughly 0° C.-30° C., the porous anodic oxide coating can be formedstably. In addition, if the temperature of the anodizing bath is roughly0° C.-5° C., a hard porous anodic oxide coating can be formed.

In the present invention, it is preferable that an electroplating bathcontaining silver ion as the metal ion be used in the electroplatingprocess, so that silver is electroplated into the pores of the porousanodic oxide coating. To this end, it is possible to use a bathcontaining, for example, 5 g/l-20 g/l of a silver salt and 10 g/l-20 g/lof a nitrate as the electroplating bath. With such a constitution,highly conductive silver will be electroplated into the pores of theanodic oxide coating, so an anodic oxide coating with a low surfaceresistance value can be formed. In addition, silver has antibacterialaction, so it is possible to give the anodic oxide coating antibacterialproperties.

In the present invention, the electroplating is performed with theelectroplating bath at a temperature of 20° C.-30° C.

In the present invention, the surface resistance value of the anodicoxide coating can be controlled by the amount of silver electroplatedinto the pores of the porous anodic oxide coating.

In the present invention, after electroplating, the anodic oxide coatingmay be colored by electroplating additional metal within the pores ofthe anodic oxide coating. In addition, the anodic oxide coating may alsobe colored after electroplating by affixing organic dyes or organicpigments within the pores of the anodic oxide coating. With such aconstitution, a design can be applied to the anodic oxide coating.

In the present invention, after the electroplating, it is preferable toseal the pores in the anodic oxide coating by performing water-vaporsealing, boiling-water sealing or low-temperature sealing. With such aconstitution, it is possible to stabilize the metal or the likeelectroplated into the pores in the anodic oxide coating.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1(A) and (B) are both cross sections showing a step in the surfacetreatment method according to the present invention.

FIGS. 2(A) and (B) are both structural drawings of an apparatus used tomeasure the surface resistance value of the anodic oxide coating formedby means of the surface treatment method according to the presentinvention.

FIGS. 3(A), (B) and (C) are all cross sections showing a step in theconventional surface treatment method.

EXPLANATION OF SYMBOLS

Porous anodic oxide coating

2 Aluminum-based material

3 Pore

4 Cell

5 Barrier layer

7 Metal

BEST MODE FOR CARRYING OUT THE INVENTION

Here follows an explanation of an embodiment of the present invention.

(Effect of the Amount of Nitric Acid Added to the Anodizing Bath on theCoating)

In carrying out the surface treatment method to which the presentinvention applies, aluminum-based material (grade: A5052P/aluminum-basedmaterial) with a thickness of 1 mm was immersed in an aqueous solutionof 3 wt. % sodium hydroxide under conditions of a temperature of 40° C.for 30 seconds to perform degreasing. Next it was rinsed with deionizedwater. Next, the aluminum-based material was immersed in an aqueoussolution of 10 wt.% nitric acid under conditions of a temperature of 15°C. for 30 seconds to perform neutralization. Next it was rinsed withdeionized water.

Next, the aluminum-based material was subjected to anodizing under theconditions given in Table 1, to form a porous anodic oxide coating onthe surface of the aluminum-based material.

TABLE 1 Item Amount of nitrate Surface resistance Samples ion added(g/l) value (MΩ) Embodiment Sample 1 21 133 Sample 2 42 125 sample 3 631.0 Sample 4 84 0.015 Sample 5 105 0.015 Sample 6 126 0.0125 ComparativeSample A 0 Infinite example

The other conditions in this anodizing treatment were as follows:

Composition of the anodizing bath Sulfuric acid 250 g/l Nitric acid0-126 g/l Dissolved aluminum 3 g/l Deionized water Remainder Anodizingbath temperature 0° C. DC current density 2.0 A/dm² Electrolysis time 30minutes

When anodizing of aluminum-based material was performed under theseconditions, a porous anodic oxide coating 1 with a thickness of 35 μmwas formed as shown in FIG. 1(A). Here, the anodizing bath containsnitrate ion together with sulfate ion which is able to form a porousanodic oxide coating 1, so by using this anodizing bath to perform theanodizing of the surface of the aluminum-based material 2, at the sametime that the porous anodic oxide coating 1 is being grown, the barrierlayer is being dissolved at the bottom 6 of the pores 3 of each cell 4.For this reason, by the time that the anodizing process is complete, thebarrier layer in the bottom of the pores 3 in the porous anodic oxidecoating 1 is thin enough to exhibit tunneling or there is no barrierlayer.

The aluminum-based material 2 thus anodized was rinsed with deionizedwater and then electroplating was performed under the followingconditions:

Composition of the electroplating bath Silver sulfate 5 g/l Nitric acid10 g/l Deionized water Remainder Voltage applied 7.0 V AC VoltageElectrolysis time 5 minutes Electroplating bath temperature 20° C.

When electroplating is performed under these conditions, by the timethat the anodizing is complete the barrier layer has already beenremoved from the bottom 6 of the pores 3 in the anodic oxide coating 1as shown in FIG. 1(A), so by simply performing electroplatingimmediately, silver or a silver compound or other metal 7 iselectroplated onto the interior of the pores 3 in the anodic oxidecoating 1.

Then, the conductivity of the anodic oxide coating formed under theabove conditions was measured. The measuring apparatus shown in FIGS.2(A) and (B) was used for this measurement. In order to evaluate thesurface resistance value of the anodic oxide coating using thismeasuring apparatus 10, first an electrode 11 for measuring resistancewas placed upon the upper surface of a piece of aluminum-based material2 measuring 50 mm×100 mm×1 mm (thick) which was subjected to anodizingand electroplating. This electrode 11 for measuring resistance consistsof a glass plate 111 measuring 20 mm×20 mm×1 mm (thick) wrapped inaluminum foil 112 with a thickness of 15 μm. In this state, theelectrode 11 for measuring resistance was pressed with a load of 3 kgagainst the aluminum-based material 2 which was subjected to anodizingand electroplating, and a DC power supply 13 applied a DC voltage of 20V between the electrode 11 and the aluminum-based material 2 which wassubjected to anodizing and electroplating. The resistance values foundfrom the current flowing at this time are presented as the surfaceresistance values in Table 1.

As shown in Table 1, in the samples according to the embodiment of thepresent invention, as the amount of nitric acid added to the anodizingbath increases, the surface resistance value of the anodic oxide coatingtended to drop to below 0.02 M. The reason for this is that as theamount of nitric acid added to the anodizing bath increases, thedissolution and removal of the barrier layer at the bottom of the poresin the anodic oxide coating is accelerated, resulting in the silver orsilver compound being more easily electroplated onto the interior of thepores of the anodic oxide coating.

In contrast, in the sample subjected to anodizing in an anodizing baththat did not contain nitrate ions (the comparative example), there was abarrier layer at the bottom of the pores in the anodic oxide coating, sono silver or silver compound was electroplated, and thus a surfaceresistance value of infinite was indicated.

(Effect of the Current Waveform at the Time of Anodizing on the Coating)

In order to study the effect of the current waveform at the time ofanodizing on the coating, as described above, aluminum-based material(grade: A5052P/aluminum-based material) with a thickness of 1 mm wassubjected to degreasing with an aqueous solution of sodium hydroxide andacid cleaning with an aqueous solution of nitric acid, and then, asshown in Table 2, anodizing was performed using various currentwaveforms including DC waveforms, AC waveforms, waveforms consisting ofDC superimposed on AC and pulse waveforms, to form a porous anodic oxidecoating on the surface of the aluminum-based material.

TABLE 2 Coating Surface Item Power thickness resistance Samples waveform(μm) (MΩ) Embodiment Sample 7 DC 15 <0.01 Sample 9 AC 10 0.2-0.3 Sample9 Superimposed 15 <0.15 Sample 10 Pulse 15 <0.18

When anodizing of aluminum-based material was performed under theseconditions, a porous anodic oxide coating 1 with a thickness of 10 μm-15μm was formed. Here, the anodizing bath contains nitrate ion, sodissolution of the barrier layer is proceeding at the bottom of thepores in the anodic oxide coating, and by the time that the anodizingprocess is complete, the barrier layer in the bottom of the pores in theporous anodic oxide coating is thin enough to exhibit tunneling or thereis no barrier layer.

The aluminum-based material thus anodized was rinsed with deionizedwater and then electroplating was performed.

Composition of the electroplating bath Silver sulfate 5 g/l Nitric acid10 g/l Deionized water Remainder Voltage applied 10 V AC voltage (0.8A/dm²) Electrolysis time 10 minutes Bath temperature 25° C.

When electroplating is performed under these conditions, by the timethat the anodizing is complete the barrier layer has already beenremoved from the bottom of the pores in the anodic oxide coating 1, soby simply performing electroplating immediately, metal is electroplatedonto the interior of the pores in the anodic oxide coating.

Then, the surface resistance value of the anodic oxide coating formedunder the above conditions was measured using the measuring apparatusdescribed in reference to FIGS. 2(A) and (B), and the results ofmeasurement are given in Table 2.

As shown in Table 2, it was confirmed that when any of DC, AC, asuperimposed waveform of DC and AC, or a pulse waveform was used, silveror a silver compound was electroplated onto the interior of the pores ofthe anodic oxide coating, and the anodic oxide coating was givenconductivity. In addition, the power waveform used at the time ofperforming anodizing is not limited to the aforementioned waveforms, butrather an imperfectly rectified waveform can also be used.

(Effect of the Silver Electroplating Time on the Coating)

In order to study the effect of the silver electroplating time on thecoating, as described above, aluminum-based material (grade:A5052P/aluminum-based material) with a thickness of 1 mm was subjectedto degreasing with an aqueous solution of sodium hydroxide and acidcleaning with an aqueous solution of nitric acid, and then, anodizingwas performed under the following conditions to form a porous anodicoxide coating on the surface of the aluminum-based material.

Composition of the anodizing bath Sulfuric acid 250 g/l Nitric acid 70g/l Dissolved aluminum 3 g/l Deionized water Remainder Anodizing bathtemperature 20° C. Current density 1.5 A/dm² Electrolysis time 30minutes

When anodizing of aluminum-based material is performed under theseconditions, the anodizing bath contains nitrate ion, so dissolution ofthe barrier layer is proceeding at the bottom of the pores in the anodicoxide coating, and by the time that the anodizing process is complete,the barrier layer in the bottom of the pores in the porous anodic oxidecoating is thin enough to exhibit tunneling or there is no barrierlayer.

The aluminum-based material thus anodized was rinsed with deionizedwater and then electroplating was performed under the conditions givenin Table 3.

TABLE 3 Item Electroplating time Surface resistance Samples (Minutes)(MΩ) Embodiment Sample 11 2 120 Sample 12 4 80 Sample 13 6 1.2 Sample 148 0.01-0.03 Sample 15 10 <0.01

Composition of the electroplating bath Silver sulfate 5 g/l Nitric acid10 g/l Deionized water Remainder Voltage applied 10 V AC voltage (0.8A/dm²) Electrolysis time 2 minutes-10 minutes Bath temperature 25° C.

When electroplating is performed under these conditions, metal iselectroplated onto the interior of the pores in the anodic oxidecoating.

Then, the surface resistance value of the anodic oxide coating formedunder the above conditions was measured using the measuring apparatusdescribed in reference to FIGS. 2(A) and (B), and the results ofmeasurement are given in Table 3.

As shown in Table 3, it was confirmed that when the electroplating timewas lengthened, the amount of silver or a silver compound electroplatedonto the interior of the pores of the anodic oxide coating increased,and accordingly the surface resistance dropped. Therefore, the surfaceresistance of the anodic oxide coating can be controlled by the amountof silver electroplated onto the pores of the porous anodic oxidecoating.

(Effect of the Anodizing Bath Composition on the Coating)

In order to study the effect of the anodizing bath composition on thecoating, as described above, aluminum-based material (grade:A5052P/aluminum-based material) with a thickness of 1 mm was subjectedto degreasing with an aqueous solution of sodium hydroxide and acidcleaning with an aqueous solution of nitric acid, and then, as shown inTable 4, anodizing was performed using an anodizing bath of variouscompositions wherein the amounts of magnesium nitrate and nitric acidadded were varied, to form a porous anodic oxide coating on the surfaceof the aluminum-based material.

TABLE 2 Anodizing bath composition Sulfuric acid 200 g/l Surface ItemMagnesium Nitric acid resistance Samples nitrate (g/l) (g/l) (MΩ)Embodiment Sample 16 10 0 0.3 Sample 17 20 0 0.1 Sample 18 30 0 0.05Sample 19 30 42 <0.01 Sample 20 30 84 <0.01

Composition of the anodizing bath Sulfuric acid 200 g/l Nitric acid 0-84g/l Magnesium nitrate 10-30 g/l Dissolved aluminum 3 g/l Deionized waterRemainder Anodizing bath temperature 20° C. Current density 1.5 A/dm²

When anodizing of aluminum-based material was performed under theseconditions a porous anodic oxide coating 1 with a thickness of 15 μm wasformed. Here, the anodizing bath contains nitrate ion, so dissolution ofthe barrier layer is proceeding at the bottom of the pores in the anodicoxide coating, and by the time that the anodizing process is complete,the barrier layer in the bottom of the pores in the porous anodic oxidecoating is thin enough to exhibit tunneling or there is no barrierlayer.

The aluminum-based material thus anodized was rinsed with deionizedwater and then electroplating was performed. The conditions for thiselectroplating were as follows:

Composition of the electroplating bath Silver sulfate 5 g/l Nitric acid10 g/l Deionized water Remainder Voltage applied 10 V AC voltage (0.8A/dm²) Electrolysis time 10 minutes Bath temperature 25° C.

When electroplating is performed under these conditions, metal iselectroplated onto the interior of the pores in the anodic oxidecoating.

Then, the surface resistance value of the anodic oxide coating formedunder the above conditions was measured using the measuring apparatusdescribed in reference to FIGS. 2(A) and (B), and the results ofmeasurement are given in Table 4.

As shown in Table 4, it was confirmed that when the amount of magnesiumnitrate (a nitrate) and nitric acid added increased, the surfaceresistance value of the anodic oxide coating tended to decrease. Thereason for this is that as the amount of nitrate ion added to theanodizing bath increases, the dissolution and removal of the barrierlayer at the bottom of the pores in the anodic oxide coating isaccelerated, resulting in the silver or silver compound being moreeasily electroplated onto the interior of the pores of the anodic oxidecoating.

Other Embodiments

Note that while the above embodiments are examples wherein nitrate ionis added to an anodizing bath containing sulfate ion as the ion able toform a porous anodic oxide coating, examples of other ions able to forma porous anodic oxide coating include not only sulfate ion and otherinorganic ions, but also oxalate ion and other organic ions, and theseions may be included in an anodizing bath to which is added nitrate ion,and this bath may be used.

In addition, the metal that is electroplated onto the interior of thepores in the anodic oxide coating by electroplating is not limited tosilver, but cobalt, nickel, tin or other metals may also beelectroplated. Moreover, after silver or other metal is electroplatedonto the interior of the pores in the anodic oxide coating byelectroplating, cobalt, nickel, tin or other metals may also beadditionally electroplated thereupon in order to color the anodic oxidecoating. In addition, the anodic oxide coating may also be colored afterelectroplating by affixing organic dyes or organic pigments or otherknown organic colorants within the pores in the anodic oxide coating.With ouch a constitution, a design can be applied to the anodic oxidecoating.

Furthermore, the aluminum-based material subjected to the surfacetreatment according to the present invention may be subjected towater-vapor sealing wherein it is exposed to water vapor; thealuminum-based material subjected to the surface treatment according tothe present invention may be subjected to boiling-water sealing whereinit is immersed in boiling deionized water or nickel acetate at atemperature of roughly 80° C.; or the aluminum-based material subjectedto the surface treatment according to the present invention may besubjected to low-temperature sealing wherein it is immersed in anaqueous solution of nickel fluoride at a temperature of roughly 40° C.,thereby preferably sealing the pores of the anodic oxide coating. Withsuch a constitution, it is possible to stabilize the metal or the likeelectroplated into the pores in the anodic oxide coating.

Industrial Applicability

As explained in the foregoing, according to the present invention,nitrate ion is added to an anodizing bath containing sulfuric acid andthe like, so when aluminum-based material is anodized, an anodic oxidecoating with the barrier layer removed from the bottom of the pores canbe produced. Therefore, it is possible to electroplate silver or othermetal upon the inside of the pores in the anodic oxide coating withoutperforming complex and time-consuming operations to remove the barrierlayer from the bottom of the pores in the anodic oxide coating. For thisreason, it is possible to use electroplated metal to form anodic oxidecoatings with new functions such as conductivity or wear resistance onthe surface of aluminum-based materials with good productivity. Thus,since the anodic oxide coating upon which metal is deposited isconductive, the aluminum-based material upon which this anodic oxidecoating is formed can be have electrostatic functions and can be usedfor crack-resistant jig and tool components for semiconductormanufacture, computer-related components, electronic components and thelike. In addition, since a hard anodic oxide coating electroplated withmetal has sliding properties, lubricant properties, wear resistance andheat resistance, it is possible to manufacture bearing members, rotarysliding components, sliders, pistons and the like from aluminum-basedmaterials upon which this anodic oxide coating is formed.

In addition, since an anodic oxide coating with silver or a silvercompound electroplated thereupon has antibacterial properties, varioustypes of antibacterial products can be manufactured from aluminum-basedmaterials upon which this anodic oxide coating is formed.

What is claimed is:
 1. A surface treatment method for aluminum-based materials comprising the steps of: anodizing an aluminum-based material consisting of aluminum or aluminum alloy in an anodizing bath comprising an amount of nitrate ion together with at least one ion selected from among an organic acid ion or an inorganic acid ion able to form a porous anodic oxide coating, thereby forming a porous anodic oxide coating on the surface of said aluminum-based material, controlling the amount of the nitrate ion in the anodizing bath to form a desired thickness of a barrier layer portion defining a bottom of each pore of the porous anodic oxide coating; and then, electroplating said aluminum-based material in an electroplating bath comprising metal ion so that metal is electroplated from said electroplating bath into the pores in said porous anodic oxide coating, thereby giving said anodic oxide coating conductivity; wherein said anodizing bath comprises 100-300 g/l of sulfuric acid and 60-140 g/l of nitric acid, and 10-100 g/l of a nitrate.
 2. The surface treatment method according to claim 1, wherein the metal ion in the electroplating bath comprises a silver ion.
 3. The surface treatment method according to claim 1, wherein after said electroplating, a different metal is affixed within the pores in said anodic oxide coating to color said anodic oxide coating.
 4. The surface treatment method according to claim 1, wherein after said electroplating, an organic colorant is affixed within the pores in the anodic oxide coating to color said anodic oxide coating.
 5. The surface treatment method according to claim 1, wherein after said electroplating, water vapor sealing is performed to seal the pores in the anodic oxide coating.
 6. The surface treatment method according to claim 1, wherein after said electroplating, boiling water sealing is performed to seal the pores in the anodic oxide coating.
 7. The surface treatment method according to claim 1, wherein after said electroplating, low temperature sealing is performed to seal the pores in the anodic oxide coating. 